Karri Kuoppamaki tells me T-Mobile has major plans for Massive MIMO in frequencies below 2300 MHz, which are designated for FDD (Frequency Division Duplexing.) TDD Massive MIMO has been shipping by the tens of thousands since 2016, but FDD is still only in trials.
Massive MIMO was developed for TDD, which sends both upstream and downstream in the same frequencies. FDD uses separate frequencies, which makes it harder for the phone to provide the base station information it needs for best performance.
TDD performance is estimated at 3X (China Mobile,) 5-7X (India,) and
up to 10X (Softbank Japan.) Terrain has an enormous effect, so speeds are very different within and across networks.
T-Mobile is seeing somewhat lower rates, as expected.